The invention is more particularly, but not exclusively, concerned with heat and moisture exchangers (HMEs) of the kind connected to a patient breathing device.
Where a patient breathes through a tube inserted in the trachea, such as a tracheostomy or endotracheal tube, gas flow to the bronchi is not warmed and moistened by passage through the nose. Unless the gas is warmed and moistened in some way it can cause damage and discomfort in the patient's throat. The gas can be conditioned by a humidifier in the ventilation circuit but, most conveniently, a heat and moisture exchange device (HME) is used. HMEs are small, lightweight devices including one or more exchange elements, such as of a paper or foam treated with a hygroscopic substance. When the patient exhales, gas passes through the exchange element and gives up a major part of its heat and moisture to the element. When the patient inhales, gas passes through the exchange element in the opposite direction and takes up a major part of the heat and moisture in the exchange element so that the gas inhaled by the patient is warmed and moistened. These HMEs are low cost and disposable after a single use so do not require cleaning or present any cross contamination risk. They can be connected in a breathing circuit or simply connected to the machine end of a tracheal tube and left open to atmosphere where the patient is breathing spontaneously.
HMEs are sold by Smiths Medical International Limited of Hythe, Kent, England under the Thermovent name (Thermovent is a registered trade mark of Smiths Medical International Limited), by Hudson RCI AB under the TrachVent name (TrachVent is a registered trade mark of Hudson RCI AB), by DAR, Medisize, Intersurgical and other manufacturers. HMEs often include an oxygen inlet port to which an oxygen supply tube can be connected. This enables supplementary oxygen to be administered to the patient via the HME. Advantageously, the oxygen port opens on the side of the HME element remote from the patient so that the oxygen has to pass through the HME element before reaching the patient. Examples of HMEs with oxygen supply ports are described in GB 2391816, WO 01/72365, U.S. Pat. No. 5,505,768, SE 516666, U.S. Pat. No. 3,881,482, DE 20302580, DE 20114355U, WO 97/01366, US 2002/0157667, U.S. Pat. No. 6,422,235, EP 1208866 and U.S. Pat. No. 4,971,054.
Because the HME is often connected to the end of a tracheal tube it is desirable that its construction be as compact as possible.